1. Field of the Invention
The invention relates to a nematic liquid crystal compound having a lactone ring, a liquid crystal composition, and a liquid crystal display device.
2. Related Art
A liquid crystal display device is classified, depending on the display mode thereof into such modes as DS (dynamic scattering), TN (twisted nematic), GH (guest host), STN (super twisted nematic), IPS (in-plane switching), VA (vertical alignment) and OCB (optically compensated bend). A liquid crystal composition contained in the liquid crystal display devices desirably has some or all of the following common characteristics in all the display modes.
(1) The composition is stable to external environmental factors, such as water; air, heat and light.
(2) The composition exhibits a liquid crystal phase in a wide temperature range centering around room temperature.
(3) The composition has a small viscosity.
(4) The composition can decrease a driving voltage upon driving the display device.
(5) The composition has an optimum dielectric anisotropy (Δ∈).
(6) The composition has an optimum optical anisotropy (Δn).
However, such a liquid crystal compound has not yet been found that satisfies all of characteristics (1) to (6) by a sole compound. Therefore, a liquid crystal composition is often obtained by mixing several kinds (e.g., a couple dozen kinds) of liquid crystal compounds. Accordingly, the liquid crystal compounds used as components of the composition necessarily have good compatibility with each other. A liquid crystal display device capable of being used in various environments, such as a very low temperature, has been demanded in recent years, and liquid crystal compounds exhibiting good compatibility at a very low temperature are thus also demanded.
In recent years, among the aforementioned display modes, such modes as IPS, VA and OCB are receiving attention as a display mode capable of overcoming a narrow viewing angle of a liquid crystal display device, which is the biggest problem of a liquid crystal display device. In particular, liquid crystal display devices of the VA mode and the IPS mode among these modes are being developed earnestly since it has excellent responsivity in addition to the wide viewing angle, and is capable of providing high-contrast display. The characteristics of the liquid crystal composition used in the liquid crystal display device of these modes reside in a negative dielectric anisotropy. It has been known that a liquid crystal composition having a large negative dielectric anisotropy can decrease the driving voltage of a liquid crystal display device containing the liquid crystal composition (as described in M. F. Leslie, Mol. Cryst. Liq. Cryst., vol. 12, p. 57 (1970)). Accordingly, liquid crystal compounds as the constitutional components of the liquid crystal composition are also demanded to have a larger negative dielectric anisotropy.
As a component of a liquid crystal composition having a negative dielectric anisotropy, various liquid crystal compounds where hydrogen at a lateral position of a benzene ring is replaced by fluorine have been investigated (as described, for example, in Japanese Patent No. 2,811,342 and JP H2-4725 A/1990). The following compound (c) (wherein R and R′ are alkyls) has been reported, for example.
However, the compounds exemplified by the compound (c) have a negative dielectric anisotropy, but the extent thereof is not necessarily large in some cases, and therefore, the compounds are not sufficient in some cases for decreasing the driving voltage of the liquid crystal display devices of the VA mode and the IPS mode.
As a liquid crystal compound having a large negative dielectric anisotropy, the compound (d) has also been reported (as described in JP S59-10557 A/1984). The compound (d) (wherein R and R′ are alkyls) has a considerably large negative dielectric anisotropy, but is not necessarily sufficient in chemical and physical stability.

Various liquid crystal compounds having a lactone ring have been investigated (as described, for example, in JP 2004-292774 A/2004, JP S59-164788 A/1984, JP 2000-8040 A/2000, JP H9-110856 A/1997, JP H7-72442 A/1995, JP H7-133244 A/1995, JP H3-20269 A/1991, Japanese Patent No. 2,974,805, Japanese Patent No. 2,974,824, Japanese Patent No. 2,974,827 and JP H3-128371 A/1991).
For example, JP 2004-292774 A/2004 and JP S59-164788 A/1984 disclose nematic liquid crystal compounds having an aromatic ring as a ring adjacent to the lactone ring.
JP H9-110856 A/1997, JP H17-72442 A/1995, JP H7-133244 A/1995 and JP H3-128371 A/1991 disclose liquid crystal compounds having a lactone ring. However, the purpose of these compounds is a ferroelectric liquid crystal, and compounds having been tested for the advantages thereof through the working examples are liquid crystal compounds having an aromatic ring or a heterocyclic ring adjacent to the lactone ring. These compounds tend to have an increased optical anisotropy and have a problem upon use as a nematic liquid crystal.
JP H3-20269 A/1991 discloses a liquid crystal compound having a lactone ring used for a ferroelectric liquid crystal, but the ring adjacent to the lactone ring is bonded through —COO—, —O— or —CH2O—. Therefore, the compound is insufficient in chemical and physical stability and has a problem upon using as a nematic liquid crystal.
JP 2000-8040 A/2000 discloses a compound apparently similar to the compounds of the invention, but the compound is disclosed merely as a synthesis intermediate.
Under the circumstances, such a liquid crystal compound is demanded that has a large negative dielectric anisotropy and that simultaneously has a suitable optical anisotropy and a high stability.
The term “liquid crystal compound” referred to herein is a generic term for a compound having a liquid crystal phase, such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a liquid crystal composition. The term “nematic liquid crystal compound” referred herein is a generic term for a compound having a nematic phase, and also for a compound having no nematic phase but being useful as a component of a nematic liquid crystal composition.